It is well known by those skilled in the art that the performance of a data communication system may be significantly improved by using one or more equalizers in the receiver of the data communications system. Typically equalizers eliminate most of the distortion caused by the transmission channel coupling a transmitter and a receiver. Further the equalizer attempts to whiten noise that is coupled to the channel from a variety of sources.
Those skilled in the art appreciate that a combination of a nonlinear feedback filter and a linear feedforward filter, a decision feedback equalizer ("DFE"), typically has performance that exceeds that obtained via a linear equalizer ("LEQ") having no feedback. However it is also known that a DFE is not a suitable technique when the decision process incurs delay, since most of the intersymbol interference ("ISI") energy typically occurs within the first few symbol intervals after a pulse has been received. Hence a delay in the feedback path eliminates the ability of the DFE to cancel ISI up to that amount of delay. For example a DFE can't be used with trellis coded modulation ("TCM") because of the delay that is inherent to the TCM decision process. One method to obtain performance nearly equivalent to a DFE is to use a Tomlinson-Harashima precoder ("precoder") where a feedback filter is in the transmitter rather than in the receiver. Because the precoder is in the transmitter, the delays that cause problems with TCM do not occur with a precoder/LEQ combination.
Precoders are typically initialized during a training mode when a training signal is sent from a transmitter to a receiver having a DFE. During the training mode, DFE coefficients are generated and then transferred via a return channel or back channel to the communication transmitter. The transferred DFE coefficients are then placed in a precoder feedback filter and are typically referred to as precoder coefficients or precoder taps. However once the data communication system goes from the training mode into a data mode, the precoder coefficients in prior art systems are fixed and may only be changed or updated by going back to the training mode.
There are some communication channels that have time-varying impairments such as time-varying changes in frequency response and in noise characteristics. For example, twisted pair loops in telecommunication channels that are undergoing environmental changes, such as temperature swings, may have significant distortion changes. The performance changes in a data communication system, having such time-varying impairments, has been pointed out by Rockwell in standards contribution T1E1.4/98-060, December 1997. Aerial cables, for example, may encounter large temperature swings. Such temperature swings cause changes in the resistance of the cable thereby reducing the effectiveness of the precoder. Hence a precoder having fixed coefficients can be less effective than a DFE when a communication channel has time-varying impairments.
It became apparent to those working with precoders, such as participants in the T1E1.4 work project on HDSL2, (Single Pair High-bit-rate Digital Subscriber Line) that a precoder update method during the data mode would alleviate performance degradation. One such method was suggested by Cicada Semiconductor in standards contribution T1E1.4/98-217, June 1998. Cicada implied that it was possible to develop a new set of precoder taps and periodically send them back to the receiver, replacing the previous precoder coefficients. Details of the Cicada method were not then and are not now publicly available nor available to the named inventors of the present invention. It was also questionable that the described technique actually worked since there was an unexplained 2 dB error in the contribution.
Because the data bandwidth on a back channel is limited there are restrictions on how often a complete set of coefficients may be sent from the receiver to the transmitter. Therefore it may not be possible to transfer a complete set of coefficients in a timely manner. In a typical application a precoder may have between 100 and 200 coefficients of 16 to 24 bits. For example, in an HDSL2 transmitter having a precoder, the precoder coefficient exchange protocol provides 22 bits for each coefficient. Hence there needs to be a method, apparatus and system for updating precoder coefficients that efficiently uses the typically limited bandwidth of a back channel.